I think boiler applies to a certain type of container where the water is not only heated, it is converted to steam, so I don't think it can be translated as "calentador de agua".
The word itself refers to the process of boiling...don't you agree?

You’re right Sandra. I guess when I saw the word boiler the picture that came to my mind was some kind of kettle, which in Venezuela we call “tetera”. Yet, I do not like the idea of limiting such device for tea; since water can be boiled for many purposes, so I used the term “calentador de agua”. But you are absolutely right in pointing out that a boiler is more than that. Thanks for your observation.